I have read that they are "resistant" to NTD due to stronger immune systems, but i have also read that they can catch it. I was researching this a few weeks ago and wasn't able to come up with any real proof, only anecdotal evidence along the lines of "i had a tank full of neons and cardinals and all the neons slowly wasted away with symptoms consistent with NTD but the cardinals were fine and never caught it", that sort of thing.
The other thing to consider is that there is apparently a "false neon tetra disease" which is bacterial instead of parasitic and can be treated. Perhaps cardinals are resisitant to the true NTD but not the false? Pure conjecture on my part.
